The invention relates to a method of checking the authenticity of documents made of a non-conducting material, such as paper or plastic, and having a physical characteristic of which the value randomly changes over a number of sub-areas of a checking area, each of said documents further comprising a digital mark which is characteristic for the distribution of said value over said sub-areas. Such documents are e.g. banknotes, passports, credit cards, bonds or other security papers. In some cases, the packing foil in which a product is enveloped, is also to be considered as a document of which the authenticity must be checked.
It is known to check the authenticity of such documents by scanning the checking-area of such documents for detecting the distribution of said value over said sub-areas, producing a digital scanning signal which is characteristic for such distribution as scanned, reading off said digital mark on said document and producing a digital mark-signal which is characteristic for the digital mark as read off, and comparing said scanning-signal with said mark-signal. In such methods the scanning is conducted by photo-electric means for detecting the distribution of the darkness of the surface of the document. It is also known per se to scan by means of a magnetic head for detecting the distribution of magnetic particles in the document.
An adapted scanning method, in order to ensure a maximum of security, has to combine an accurate repetitivity and a high resolution, i.e. capability to follow sharp variations over short distances. When a scanning system cannot follow such sharp variations, it will only be capable to distinguish two coarsely different patterns from each other, and will not be capable to distinguish two slightly different documents from each other. As a result, the documents are less individualized and falsification is easier.
Besides a high resolution, an accurate repetitivity of the scanning signal is also important. Because of magnetic prehistory or disturbing of magnetic fields or deformations of the document, the repetitivity of a magnetic scanning system is not ensured, whereas loss of brightness or colour or other damage of the document will disturb the repetitivity of a photo-electric scanning system. However, if the scanning system is not sufficiently repetitive, the risk that an authentic document would produce a slightly different scanning-signal and be identified as a falsification is not negligible. In these circumstances it is not possible to submit the scanning-signal to a severe criterion of identity with a given signal, and it has no sense to improve the resolution as long as the repetitivity is not also improved, and inversely.